Fabricated titanium article having improved corrosion resistance

ABSTRACT

A titanium article having improved corrosion resistance resulting from a direct or indirect attachment of a platinum group metal or alloy thereof or incorporation of this metal or alloy thereof into a minor surface portion of the article.

RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/253,437, filed on Sep. 25, 2002, now U.S. Pat. No.6,607,846.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a titanium article wherein improved corrosionresistance is achieved by the integral incorporation of a platinum groupmetal or alloy thereof directly or indirectly attached to a minorsurface portion of said article.

2. Description of the Prior Art

Titanium, being a reactive metal, relies on the formation and stabilityof a surface oxide film for corrosion resistance. Under stableconditions, titanium can demonstrate remarkable corrosion resistantbehavior. The reverse is also true, however, in that when the film isdestabilized, extremely high corrosion rates may result. Theseconditions of instability are generally at the two extremes of the pHscale. Strongly acidic or alkaline solutions can create instability inthe titanium oxide film.

Typically, in accordance with prior art practice, when using titanium inan area of uncertain oxide film stability, alloying elements have beenadded to the titanium to enhance the oxide film stability, thusincreasing its effective usefulness at the pH extremes. This practicehas proven most effective for the acid end of the pH scale. Severalalloying elements have shown success in this regard, such as molybdenum,nickel, tantalum, niobium and precious metals. Of this group, theplatinum group metals (PGM) offer far and away the most effectiveprotection against corrosion. The platinum group metals are platinum,palladium, ruthenium, rhodium, iridium and osmium.

Stern et al. demonstrated this in 1959 in a paper titled. “The Influenceof Noble Metal Alloy Additions on the Electrochemical and CorrosionBehavior of Titanium”. They found that as little as 0.15% Pd or Ptalloying additions greatly enhanced the stability of the oxide film ontitanium, and thus the corrosion resistance, in hot reducing acidmedium. Consequently, for many years the ASTM grade 7 titanium(Ti-0.15Pd) has been the standard material for use in severe corrosiveconditions where unalloyed titanium is subject to corrosion. Morerecently, ASTM grade 16 (Ti-0.05Pd) and grade 26 (Ti-0.1 Ru) have beenused as direct replacements for grade 7 because they are more economicaland provide a level of corrosion resistance close to that of grade 7.Thus, they tend to be considered equivalent in less drastic corrosionapplications.

The mechanism of protection afforded by platinum group metal additionsto titanium is one of increased cathodic depolarization. The platinumgroup metals afford a much lower hydrogen overvoltage in acidic media,thereby increasing the kinetics of the cathodic portion of theelectrochemical reaction. This increased kinetics translates to a changein the slope of the cathodic half reaction, leading to a more noblecorrosion potential for the titanium. The active/passive anodic behaviorof titanium allows for a small shift in corrosion potential(polarization) to effect a large change in the corrosion rate.

Prior work on the polarization behavior of titanium was performed byStern and Wissenberg in 1959. In this work, titanium was galvanicallycoupled to other metals to observe the effects on the corrosion rate oftitanium in sulfuric acid media. The researchers found that titaniumcoupled to platinum could exhibit a reduction in corrosion rate of up to100 fold, as seen in the present invention. However, they were notdirectly attaching the platinum onto the titanium and thus did notrealize the extraordinary benefits seen in the present invention. Inorder for Stern and Wissenberg to achieve a 100 fold reduction incorrosion, they found that the surface area of the platinum had to be 4times the area of the titanium. Thus, their surface area ratio of Ti/PGMwas ¼. In fact, at a 35/1 Ti/PGM surface area ratio, the authors saw nobenefit from the platinum coupling whatsoever. This was clearly notadvantageous in terms of cost and thus, it is presumed the researchersthen pursued alloying as the means of enhanced environmental behavior,as disclosed in U.S. Pat. No. 3,063,835. In the present invention usinga direct or indirect attachment or integral incorporation practice, the100 fold corrosion rate reduction is observed at all Ti/PGM surface arearatios, even to ratios thousands of times larger than Stern andWissenberg's.

Although the above-described prior art practices are effective forenhancing the corrosion resistance of titanium in severe corrosiveconditions, alloying additions of precious metals and especially theplatinum group metals are extremely expensive.

SUMMARY OF THE INVENTION

The invention of the instant application provides, in place of bulkalloying, a relatively low cost and easy to apply practice for achievingimproved corrosion resistance of titanium subjected to severe corrosiveapplications, and thus is advantageous in this regard when compared tothe prior art practices discussed above.

In accordance with the invention, it has been determined that a simpledirect or indirect application or integral incorporation of a smallamount of a platinum group metal (PGM) or alloy thereof onto thetitanium surface can protect a large surface area of the titanium. ThePGM or alloy thereof is not intentionally alloyed with the titanium butinstead, plated, resistance welded, fusion welded, mechanicallyattached, or vapor deposited to achieve direct or indirect attachment orintegral incorporation to a minor surface portion of a titanium article.In accordance with the invention, a titanium article having improvedcorrosion resistance comprises a titanium substrate having a direct orindirect attachment to a minor surface portion thereof, or integralincorporation of a platinum group metal or alloy thereof present in anamount effective to produce an article that exhibits better corrosionresistance than the article without the attachment or integralincorporation. The PGM attachment or integral incorporation is presentin an area thereof less than 1% (excluding 0) of the article surfacearea to be protected. A ratio of the surface area of the article to thearea of the attachment or integral incorporation of greater than 10 orgreater than 50 to 10000 may be used in accordance with the invention.Although any desired practice may be used for the direct or indirectattachment, preferred practices include plating, resistance welding,fusion welding, mechanically fastening, and vapor deposition. The PGM oralloy thereof can also be integrally incorporated into the titaniumarticle, becoming part of the article. This could be accomplished by anynumber of means including, but not limited to, welding an appliqué intoan article thus making it part of said article, using the PGM or alloythereof as weld filler metal during fabrication of the article or lateras an addition or repair or enhancement of an existing article, or usingthe PGM or alloy thereof as a pre-fabricated component that becomes thenattached to the article. The preferred PGM is platinum and the preferredPGM alloy is an alloy containing 0.3-15% Pd or 0.3-15% Pt. The level ofPGM in grade 7 titanium (0.15% Pd) was shown to be insufficient to offerany benefit when used as an appliqué on grade 2 titanium (see Table 3).An effective platinum group alloy for use in the practice of theinvention may include Ti-0.3 to 15% Pdor Pt or 1% Pd or Pt.

DESCRIPTION OF THE PREFERRED EMBODIMENTS AND SPECIFIC EXAMPLES

In experimental work leading to the invention, general corrosion testingwas performed with varying surface area ratios with excellent results.The polarization effect (change in corrosion potential) was observedover a significant distance. As shown in Tables 1 and 2, the practice ofthe invention can be more effective than either grade 16 or grade 26 inboiling HCl at substrate to appliqué surface area ratios as great as5000. Table 3 demonstrates the effectiveness of incorporating a Ti-PGMalloy as weld wire onto a titanium grade 2 substrate.

TABLE 1 Corrosion Rates in Hydrochloric Acid Area Ratio Max. DistanceCorrosion Solution (Ti Gr2/ from PGMA Rate Test Material (@ boiling)PGM) (in) (mpy) ASTM Grade 2 5% HCl — — −1000 ASTM Grade 7 5% HCl — —4.7 ASTM Grade 16 5% HCl — — 5.4 ASTM Grade 26 5% HCl — — 12.2 Grade 2with Pt¹ 5% HCl  66/1 2 4.3 Grade 2 with Pt² 5% HCl 110/1 2 4.9 Grade 2with Pt² 5% HCl 220/1 2 5.4 Grade 2 with Pt² 5% HCl 440/1 2 4.9 Grade 2with Pt² 5% HCl 440/1 4 5.2 Grade 2 with Pt² 5% HCl 2000/1  8 4.0 Grade2 with Pt² 5% HCl 5100/1  10  4.2 Grade 2 with Pt¹ 5% HCl 1000/1  4 4.3Grade 2 with Pd² 5% HCl  66/1 2 4.7 Grade 2 with Rh² 5% HCl  66/1 2 5.6Footnotes: ¹PGM was plated onto surface. ²PGM was resistance welded ontosurface as a sheet.

TABLE 2 Corrosion Rates for PGM Alloy Appliqué in Hydrochloric Acid AreaRatio Corrosion Solution (Ti Gr2/ Rate Test Material (@ boiling) PGMAlloy) (mpy) ASTM Grade 2 5% HCl — ˜1000 ASTM Grade 7 5% HCl — 4.7 ASTMGrade 16 5% HCl — 5.4 ASTM Grade 26 5% HCl — 12.2 Grade 2 with Ti-0.5%Pt 5% HCl 125/1 9.5 Grade 2 with Ti-1% Pt 5% HCl 125/1 5.8 Grade 2 withTi-1% Pt 5% HCl 250/1 6.9 Grade 2 with Ti-1% Pt 5% HCl 500/1 1060 Grade2 with Ti-0.5% Pd 5% HCl 125/1 10.9 Grade 2 with Ti-1% Pd 5% HCl 125/14.8 Grade 2 with Ti-1% Pd 5% HCl 250/1 8.9 Grade 2 with Ti-1% Pd 5% HCl500/1 940

TABLE 3 Corrosion Rates for PGM alloy Weld Wire Incorporation in HClArea Ratio Corrosion Solution (TI Gr2/ Rate Test Material (@ boiling)PGM Alloy) (mpy) ASTM Grade 2 5% HCl — ˜1000 ASTM Grade 7 5% HCl — 4.7Grade 2 with Ti-0.15% Pd 5% HCl 125/1  153 Grade 2 with Ti-0.5% Pt 5%HCl 32/1 6.8 Grade 2 with Ti-1% Pt 5% HCl 125/1  5.9 Grade 2 withTi-0.5% Pd 5% HCl 32/1 5.6 Grade 2 with Ti-1% Pd 5% HCl 83/1 6.2

Likewise, the corrosion rates can be lowered in oxidizing acids as well.This is demonstrated in Table 4 in concentrated nitric acid. In thisinstance, the titanium with the Pt appliqué actually performed betterthan documented rates for grade 7.

TABLE 4 Corrosion Rates in Nitric Acid Corrosion Test Material SolutionArea Ratio Rate (mpy) Comments ASTM Grade 2 40% @ Boiling — 24.1 96 Hr.Exposure ASTM Grade 7 40% @ Boiling — 25 From data archive ASTM Grade 1240% @ Boiling — 15 From data archive Grade 2 with Pt 40% @ Boiling 66/16.7 96 Hr. Exposure

It was also determined in crevice corrosion testing that the titaniummetal within a crevice can be effectively protected by application ofthe PGM appliqué on the titanium substrate outside of the crevice, asshown in Table 5. Once again, the results confirmed that the grade 2titanium with a PGM appliqué could exhibit equivalent corrosion behavioras that of grade 7 titanium.

TABLE 5 Crevice Corrosion Results Max. Distance Severity of Area fromPGMA Crevice Test Material Solution Ratio (in.) Corrosion ASTM Grade 25% NaCl, pH 3 — — Moderate Attack ASTM Grade 7 5% NaCl, pH 3 — — NoAttack ASTM Grade 5% NaCl, pH 3 — — Minor Attack 12 Grade 2 with Pt 5%NaCl, pH 3 120/1 3 No Attack Grade 2 with Pt 5% NaCl, pH 3 120/1 5 NoAttack ASTM Grade 2 5% NaCl, pH 1 — — Severe Attack ASTM Grade 7 5%NaCl, pH 1 — — No Attack ASTM Grade 5% NaCl, pH 1 — — Moderate 12 AttackGrade 2 with Pt 5% NaCl, pH 1 120/1 3 No Attack Grade 2 with Pt 5% NaCl,pH 1 120/1 5 No Attack ASTM Grade 7 5% NaCl + — — No Attack 1000 ppmFe³⁺ pH 0.5 ASTM Grade 5% NaCl + — — Severe Attack 12 1000 ppm Fe³⁺ pH0.5 Grade 2 with Pt 5% NaCl + 120/1 3 No Attack 1000 ppm Fe³⁺ pH 0.5Grade 2 with Pt 5% NaCl + 120/1 5 No Attack 1000 ppm Fe³⁺ pH 0.5

The method of application of the PGM or the alloy thereof does notaffect the performance as long as the attachment or incorporation isdirectly or indirectly attached to the titanium substrate. This effectwas observed with different PGM metals, such as platinum, palladium andrhodium. The extent of protection does vary somewhat with differentplatinum group metals and alloys thereof, depending upon the corrosivemedia; however, in all cases significant corrosion resistance wasachieved.

The strength of the polarization effect of an appliqué of PGM was testedby several means. Simultaneous tests in boiling acid utilizing variousTi/PGM area ratios were performed. In addition, samples of the same arearatio but with varying distances between the farthest edge of thetitanium test coupon and the appliqué were also studied. As an example,in one instance, the ratio was set at 250/1; however, one set of couponswere twice the length of a second set. Thus, the distance over which thePGM appliqué was forced to protect (polarize) was doubled. This distancedifference had no effect on the protective polarization of the testcoupon. In both instances, the appliqué was applied to only one side ofthe test coupon; however, the polarization effect did not suffer fromone side of the specimen to the other.

As may be seen from the experimental work above described in hotreducing acid environments, the titanium sample with the appliqué orincorporation can exhibit the same corrosion behavior as ASTM grade 7(Ti-0.15Pd).

The cost benefits of the invention over conventional practices are huge.Specifically, at only a 500/1 surface area ratio, the incremental costof the PGM appliqué over the base cost of the titanium is about $0.50/lbat a titanium thickness of 0.125 in and drops to $0.25/lb at a thicknessof 0.25 in. By contrast, the incremental cost of grade 7, which istitanium alloyed with 0.15% palladium, over grade 2 commercially puretitanium, is on the order of $15/lb. This will not change with metalthickness since it is an alloying addition, so at 0.125 in gauge, theappliqué offers roughly a 96% cost reduction while at 0.25 in titaniummetal thickness, the cost reduction is more on the order of 98%.

Similarly, using a Ti-1% Pd alloy as the applique or incorporating it asweld metal into a titanium article, at a 125/1 ratio, the incrementalcosts would be about $0.13/lb. at a thickness of 0.125 in. and only$0.07/lb. at a 0.25 in. thickness. Thus, in this example, the costreductions versus use of grade 7 are greater than 99%.

The invention also provides significant advantages with respect todelivery and availability of the corrosion resistant material.Specifically, companies do not normally inventory titanium alloyscontaining a PGM due to the cost of inventorying these high cost metals.Thus, these grades tend to be less available than standard grades oftitanium that do not contain an alloyed PGM. Consequently, deliverytimes tend to be longer since manufacturers are generally required towork these melts into their melting schedule as time permits. Whereas,normal grades of titanium are in production on a routine basis andadditional melts may be added without time delays.

The invention offers great versatility in that it may be used by themanufacturer, the fabricator or at a facility of an end user.Specialized equipment or specialized skills are not necessary.

The invention may be specifically targeted to areas of process equipmentthat will be used in environments more susceptible to corrosion. Thiscould further reduce the overall cost of utilizing the invention. Inthis regard, the practice of the invention would allow for in-siturepair of existing titanium equipment that begins to suffer fromcorrosion.

The practice of the invention allows for selection of the mostappropriate PGM or PGM alloy for a specific environment in order tomaximize corrosion performance and reduce cost. This is not the casewith PGM alloyed grades of titanium where the PGM alloying addition isfixed in the alloyed article.

The term “titanium” as used herein in the specification and claimsrefers to elemental titanium, commercially pure titanium and titaniumbase alloys. The term “platinum group metals” (PGM) as used herein inthe specification and claims refers to platinum (Pt), palladium (Pd),ruthenium (Ru), rhodium (Rh), iridium (Ir), and osmium (Os). The termplatinum group metal (PGM) alloy as used herein in the specification andclaims refers to the use of an alloy whose minor constituent consists ofa PGM or an alloy thereof comprised of 2 or more platinum group metals.The term “corrosion” as used herein in the specification and claims isdefined as the chemical or electrochemical reaction between a material,usually a metal, and its environment that produces a deterioration ofthe material and its properties.

1. A titanium article having improved corrosion resistance, comprising atitanium substrate having a direct or indirect attachment to a minorsurface portion thereof, said attachment comprising a platinum groupmetal or platinum group metal alloy, or having an integral incorporationof a platinum group metal or platinum group metal alloy, wherein theplatinum group metal is present in an area less than 1% (excluding 0) ofthe surface area to be protected or any minor surface area for theplatinum group metal alloy, whereby said article exhibits bettercorrosion resistance than said article without said attachment orintegral incorporation.
 2. The titanium article of claim 1, wherein saidattachment or integral incorporation is present in an article surfacearea to an attachment or integral incorporation ratio of 10 to
 10000. 3.The titanium article of claim 1, wherein said attachment or integralincorporation is present in an article surface area to attachment orintegral incorporation ratio of 50 to
 10000. 4. The titanium article ofclaim 1, or claim 2, or claim 3, wherein said platinum group metal isplatinum.
 5. The titanium article of claim 1, or claim 2, or claim 3,wherein said platinum group metal alloy includes Ti-0.3-15% Pd orTi-0.3-15% Pt.
 6. The titanium article of claim 1, or claim 2, or claim3, wherein said platinum group metal alloy includes 1% Pd or 1% Pt.
 7. Atitanium article having improved corrosion resistance, comprising atitanium substrate having a direct or indirect attachment to or havingan integral incorporation of a minor surface portion thereof by weldingof a platinum group metal or platinum group metal alloy, wherein saidattachment or integral incorporation is present in an area less than 1%(excluding 0) of the surface area to be protected or any minor surfacearea for the platinum group metal alloy, whereby said article exhibitsbetter corrosion resistance than said article without said attachment orintegral incorporation.
 8. The titanium article of claim 7, wherein saidattachment or integral incorporation is present in an article surfacearea to attachment or integral incorporation ratio of greater than 10 to10000.
 9. The titanium article of claim 7, wherein said attachment orintegral incorporation is present in an article surface area toattachment or integral incorporation ratio of greater than 50 to 10000.10. The titanium article of claim 7, or claim 8, or claim 9, whereinsaid platinum group metal is platinum.
 11. The titanium article of claim7, or claim 8, or claim 9, wherein said platinum group metal alloycomposition range includes Ti-0.3-15% Pd or Ti-0.3-15% Pt.
 12. Thetitanium article of claim 7, or claim 8, or claim 9, wherein saidplatinum group metal alloy includes 1% Pd or 1% Pt.
 13. A titaniumarticle having improved corrosion resistance, comprising a titaniumsubstrate having a direct or indirect attachment to a minor surfaceportion thereof, said attachment comprising a plating of a platinumgroup metal or platinum group metal alloy wherein said attachment ispresent in an area less than 1% (excluding 0) of the surface area to beprotected or any minor surface area for the platinum group metal alloy,whereby said article exhibits better corrosion resistance than saidarticle without said attachment.
 14. The titanium article of claim 13,wherein said attachment is present in an article surface area toattachment ratio of greater than 10 to
 10000. 15. The titanium articleof claim 14, wherein said attachment is present in an article surfacearea to attachment ratio of greater than 50 to
 10000. 16. The titaniumarticle of claim 13, or claim 14 or claim 15, wherein said platinumgroup metal is platinum.
 17. The titanium article of claim 13, or claim14, or claim 15, wherein said platinum group metal alloy includesTi-0.3-15% Pd or Ti-0.3-15% Pt.
 18. The titanium article of claim 13, orclaim 14, or claim 15, wherein said platinum group metal alloy includes1% Pd or 1% Pt.
 19. A titanium article having improved corrosionresistance, comprising a titanium substrate having a direct or indirectattachment to a minor surface portion thereof, said attachmentcomprising a vapor deposited platinum group metal or platinum groupmetal alloy, wherein said attachment is present in an area less than 1%(excluding 0) of the surface area to be protected or any minor surfacearea for the platinum group metal alloy, whereby said article exhibitsbetter corrosion resistance than said article without said attachment.20. The titanium article of claim 19, wherein said attachment is presentin an article surface area to attachment ratio of greater than 10 to10000.
 21. The titanium article of claim 19, wherein said attachment ispresent in an article surface area to attachment ratio of greater than50 to
 10000. 22. The titanium article of claim 19, or claim 20, or claim21, wherein said platinum group metal is platinum.
 23. The titaniumarticle of claim 19, or claim 20, or claim 21, wherein said platinumgroup metal alloy includes Ti-0.3-15% Pd or Ti-0.3-15% Pt.
 24. Thetitanium article of claim 19, or claim 20, or claim 21, wherein saidplatinum group metal alloy includes 1% Pd or 1% Pt.
 25. A titaniumarticle having improved corrosion resistance, comprising a titaniumsubstrate having directly or indirectly attached to or having anintegral incorporation of a minor surface portion thereof by means of amechanical fixture comprising a platinum group metal or platinum groupmetal alloy, wherein the platinum group metal is present in an area lessthan 1% (excluding 0) of the surface area to be protected or any minorsurface area for the platinum group metal alloy, whereby said articleexhibits better corrosion resistance than said article without saidmechanical fixture.
 26. The titanium article of claim 25, wherein saidmechanical fixture is present in an article surface area to mechanicalfixture ratio of greater than 10 to
 10000. 27. The titanium article ofclaim 26, wherein said mechanical fixture is present in an articlesurface area to mechanical fixture ratio of greater than 50 to 10000.28. The titanium article of claim 25, or claim 26, or claim 27, whereinsaid platinum group metal is platinum.
 29. The titanium article of claim25, or claim 26, or claim 27, wherein said platinum group metal alloycomposition range includes Ti-0.3-15% Pd or Ti-0.3-15% Pt.
 30. Thetitanium article of claim 25, or claim 26, or claim 27, wherein saidplatinum group metal alloy includes 1% Pd or 1% Pt.
 31. The titaniumarticle of claim 25, or claim 26, or claim 27, wherein said mechanicalfixture comprises a prefabricated component containing the platinumgroup metal or platinum group metal alloy.